This thesis aims to reveal insights to the brain structures involved in generating Essential Tremor. Essential Tremor (ET) is the most common movement disorder. Its cause is often linked to the cerebellum and the thalamic target of cerebellar output--the ventral inferior medial thalamic nucleus (Vim). Psychophysical methods will be applied to examine the effect of two surgical treatments of ET, Vim thalamotomy and deep brain stimulation (DBS), on movement control and learning. Patients will perform reaching movement tasks that involve adaptation to novel force fields. Within-subject learning indices comparisons will be used to assess the effect of thalamotomy and DBS. Functional mapping requirements of the surgical treatments and our portable robotic manipulandum technology gave us unprecedented opportunity to examine the neuronal activity of human cerebellar thalamus during reaching movement. Correlation between the Vim spike activity and movement parameters will be used to delineate the role of Vim in movement control and learning. Characterizing Vim single unit property and the effect of thalamotomy and DBS in Vim will lead to better understanding of the neural mechanisms of movement control as well as the mechanism of these surgical interventions, and could potentially lead to advances in treatment of the disorder.